U.S. Pat. No. 3,367,992 describes polymerizable systems prepared from diepoxides and maleic anhydride semiesters with hydroxymethacrylates. The systems are cured with peroxides and are useful in the manufacturing of laminates or plastics. Similar systems are disclosed for use as electron beam curable binders in Deutsche Offenlegungsschrift No. 1,916,500. It is a serious disadvantage of these coating compositions that even with a high dosage of electron beams, the cured surface is very soft owing to the open chain portion of maleic acid and hydroxy(meth)acrylate respectively. This disadvantage occurs although the double bond of maleic acid is an additional crosslinking site on irradiation. The resulting higher crosslinking density has a negative influence on film flexibility.
Deutsche Offenlegungsschrift No. 2,221,091 and U.S. Pat. No. 3,683,045 disclose similar systems and state that the storage stability of the resin solution can be enhanced through addition of phenothiazine. The disadvantages of such systems as described above remain unmentioned.
Deutsche Offenlegungsschrift No. 2,164,386 describes the preparation of electron beam curable coating compositions through reaction of diepoxides with acrylic acid and subsequent reaction with diisocyanates. It is an essential property of such systems that, owing to the very small molecule obtained from the diepoxide and acrylic acid reaction, on irradiation hard but brittle films are obtained due to the high crosslinking density. When using high molecular diepoxides in order to reduce the number of double bonds, the high viscosity of such products is a disadvantage. In such case, strong dilution with monomers is necessary, which, in turn, adverssely influences the curing dose needed and film properties of te cured film. If, in variation of the above method, aliphatic diepoxides are used, the cured films, in addition, have a very low surface hardness. The presence of a great number of carboxy groups furthermore leads to insufficient chemical resistance and weather resistance.
A further disadvantage of the products of Deutsche Offenlegungsschrift No. 2,164,386 is that the reaction with the diisocyanates leads to an increase in the size of the molecule which entrains a reduction of polymerization capacity, since the number of double linkages per weight unit becomes lower. In addition, there is the hazard of premature gelation, since each molecule of diisocyanate is able to link two polycondensate chains by reaction with each 2 hydroxy or carboxy groups.